The present invention relates to an apparatus for controlling the air-fuel ratio of an internal combustion engine, and more particularly to an air-fuel ratio control apparatus which is capable of evaluating the deteriorated state of a catalytic converter for purifying exhaust gases.
Conventional processes of determining the deteriorated state of a catalytic converter for purifying exhaust gases which is disposed in the exhaust passage of an internal combustion engine are known from Japanese patent publication No. 2,526,640 and Japanese laid-open patent publication No. 7-19033, for example.
The disclosed techniques are based on the fact that when the air-fuel ratio of an air-fuel mixture to be combusted by an internal combustion engine is changed from a leaner value to a richer value or from a richer value to a leaner value, the outputs from oxygen concentration sensors that are positioned respectively upstream and downstream of a catalytic converter combined with the internal combustion engine are inverted. More specifically, under certain operating conditions of the internal combustion engine, i.e., when the power output of the internal combustion engine is increased or the fuel supplied to the internal combustion engine is cut off as disclosed in Japanese patent publication No. 2,526,640 or when certain conditions are satisfied, e.g., the load and rotational speed of the internal combustion engine are in predetermined ranges as disclosed in Japanese laid-open patent publication No. 7-19033, the air-fuel ratio is positively changed from a leaner value to a richer value or from a richer value to a leaner value. At this time, the time consumed after the output of the upstream oxygen concentration sensor is inverted until the output of the downstream oxygen concentration sensor is inverted, and the period at which output of the downstream oxygen concentration sensor is inverted are measured, and the deteriorated state of the catalytic converter is evaluated based on the measured values.
According to these techniques, when the internal combustion engine is operating under ordinary conditions, i.e., conditions without estimating the deteriorated state of the catalytic converter, the air-fuel ratio is feedback-controlled depending on the inversion of the outputs from the oxygen concentration sensors in order to keep the air-fuel ratio of the internal combustion engine in the vicinity of a stoichiometric air-fuel ratio, for thereby allowing the catalytic converter to keep an appropriate purifying capability.
According to the above process of evaluating the deteriorated state of the catalytic converter, however, it is necessary to positively change the air-fuel ratio from a leaner value to a richer value or from a richer value to a leaner value in order to evaluate the deteriorated state of the catalytic converter. Consequently, as long as the air-fuel ratio of the internal combustion engine is feedback-controlled so as to allow the catalytic converter to keep an appropriate purifying capability, it is impossible to evaluate the deteriorated state of the catalytic converter. Therefore, at the time of evaluating the deteriorated state of the catalytic converter, it is difficult for the catalytic converter to keep an appropriate purifying capability.
The applicant of the present application has proposed another technique for achieving an appropriate purifying capability of a catalytic converter (see, for example, Japanese laid-open patent publication No. 9-324681, Japanese laid-open patent publication No. 11-153051, U.S. Pat. No. 5,852,930, and U.S. patent application Ser. No. 09/153,300). According to the proposal, an exhaust gas sensor for detecting the concentration of a certain component, e.g., oxygen, of exhaust gases is positioned downstream of the catalytic converter, and the air-fuel ratio of an air-fuel mixture to be combusted by an internal combustion engine is manipulated in order to converge the output of the exhaust gas sensor to a predetermined target value.
More specifically, in order to converge the output (the detected value of the oxygen concentration) of the exhaust gas sensor to a predetermined target value (constant value), a target value (target air-fuel ratio) for the air-fuel ratio of the exhaust gases that enter the catalytic converter, or specifically the air-fuel ratio recognized from the oxygen concentration of the exhaust gases, is successively calculated according to a sliding mode control process. The air-fuel ratio of the air-fuel mixture to be combusted by an internal combustion engine is then manipulated depending on the target air-fuel ratio to achieve the appropriate purifying capability of the catalytic converter.
Since the above proposed technique is capable of stably keeping the appropriate purifying capability of the catalytic converter by controlling the air-fuel ratio as described above, it is desirable to be able to evaluate the deteriorated state of the catalytic converter while performing the above air-fuel control process.
It is therefore an object of the present invention to provide an apparatus for controlling the air-fuel ratio of an internal combustion engine, which is capable of appropriately evaluating the deteriorated state of a catalytic converter while keeping a desired purifying capability of the catalytic converter that is disposed in the exhaust passage of the internal combustion engine.
To achieve the above object, there is provided in accordance with the present invention an apparatus for controlling the air-fuel ratio of an internal combustion engine, comprising an exhaust gas sensor disposed downstream of a catalytic converter which is disposed in an exhaust passage of an internal combustion engine, for detecting the concentration of a particular component of a exhaust gas emitted from the internal combustion engine and having passed through the catalytic converter, air-fuel ratio manipulated variable generating means for sequentially generating a manipulated variable to determine the air-fuel ratio of the exhaust gas entering the catalytic converter to converge an output of the exhaust gas sensor to a predetermined target value, air-fuel ratio manipulating means for manipulating the air-fuel ratio of an air-fuel mixture to be combusted in the internal combustion engine depending on the manipulated variable, and deteriorated state evaluating means for sequentially determining the value of a deterioration evaluating linear function from time-series data of the output of the exhaust gas sensor, the deterioration evaluating linear function having variable components represented by the time-series data of the output of the exhaust gas sensor, and evaluating a deteriorated state of the catalytic converter based on the determined value of the deterioration evaluating linear function, while the air-fuel ratio of the air-fuel mixture is being manipulated by the air-fuel ratio manipulating means.
The inventors of the present invention have found that while a manipulated variable for the air-fuel ratio (e.g., a target value for the air-fuel ratio) of the exhaust gas entering the catalytic converter to converge the output of the exhaust gas sensor downstream of the catalytic converter to a given target value is being sequentially generated and the air-fuel ratio of the air-fuel mixture is being manipulated depending on the manipulated variable, when an appropriate linear function whose variable components are represented by time-series data of the output of the exhaust gas sensor, i.e., a linear function expressed as a linear coupling of the time-series data of the output of the exhaust gas sensor, is determined from the time-series data of the output of the exhaust gas sensor, the value of the linear function has a tendency to exhibit a characteristic correlation between itself and the degree to which the deterioration of the catalytic converter progresses.
For example, when the catalytic converter is substantially brand-new, the value of the linear function tends to be accumulated in the vicinity of a certain value. As the deterioration of the catalytic converter progresses, the value of the linear function tends to go away from the certain value. Stated otherwise, as the deterioration of the catalytic converter progresses, the degree to which the value of the linear function varies becomes larger.
The deteriorated state evaluating means employs the above linear function as the deterioration evaluating linear function, and sequentially determines the value of the deterioration evaluating linear function from the time-series data of the output of the exhaust gas sensor. The deteriorated state evaluating means determines the deteriorated state of the catalytic converter based on the value of the deterioration evaluating linear function.
The value of the deterioration evaluating linear function as a basis for evaluating the deteriorated state of the catalytic converter is determined from time-series data of the output of the exhaust gas sensor while the air-fuel ratio manipulating means is manipulating the air-fuel ratio of the air-fuel mixture depending on the manipulated variable that is generated by the air-fuel ratio manipulated variable generating means in order to converge the output of the exhaust gas sensor to the target value. Therefore, the value of the deterioration evaluating linear function is obtained while the air-fuel ratio is being manipulated to maintain a desired purifying capability of the catalytic converter.
Thus, the deteriorated state of the catalytic converter can be evaluated while maintaining the desired purifying capability of the catalytic converter, by evaluating the deteriorated state of the catalytic converter based on the value of the deterioration evaluating linear function.
In the present invention, the above tendency of the value of the deterioration evaluating linear function depending on the degree to which the deterioration of the catalytic converter progresses is likely to be clear when the manipulated variable is generated by the air-fuel ratio manipulated variable generating means according to a sliding mode control process as one feedback control process. If the air-fuel ratio manipulated variable generating means comprises means for generating the manipulated variable according to the sliding mode control process, then the deterioration evaluating linear function which is highly correlated to the deteriorated state of the catalytic converter is closely related to a switching function used in the sliding mode control process. Therefore, a linear function determined depending on the switching function is preferably used as the deterioration evaluating linear function.
More specifically, the sliding mode control process used by the air-fuel ratio manipulated variable generating means employs a linear function whose variable components are represented by time-series data of the difference between the output of the exhaust gas sensor and the target value, for example, as the switching function. If this switching function is used in the sliding mode control process, then the deterioration evaluating linear function preferably comprises a linear function which has coefficients of variable components that are identical to coefficients of the variable components of the switching function. The linear function may be the switching function itself for the sliding mode control process.
With the linear function determined depending on the switching function for the sliding mode control process being used as the deterioration evaluating linear function, the correlation between the value of the deterioration evaluating linear function and the deteriorated state of the catalytic converter is manifested, allowing the deteriorated state of the catalytic converter to be evaluated appropriately based on the value of the deterioration evaluating linear function.
Even if the manipulated variable is generated by any of various other feedback control processes including a PID (proportional plug integral plus derivative) control process, when a linear function similar to the switching function for the sliding mode control process is determined as the deterioration evaluating linear function, it is possible to provide a correlation of the above tendency between the value of the deterioration evaluating linear function and the deteriorated state of the catalytic converter.
According to the present invention, the deteriorated state evaluating means preferably comprises means for determining data representing the degree to which time-series data of the value of the deterioration evaluating linear function vary, as a deterioration evaluating parameter from the time-series data of the value of the deterioration evaluating linear function, and evaluating the deteriorated state of the catalytic converter based on the value of the determined deterioration evaluating parameter.
Specifically, as the deterioration of the catalytic converter progresses, the value of the deterioration evaluating linear function is liable to change away from a certain value, resulting in a large variation of the value of the deterioration evaluating linear function. According to the present invention, the data representing the degree to which the time-series data of the value of the deterioration evaluating linear function vary is used as the deterioration evaluating parameter, and the deterioration evaluating parameter is determined from the time-series data of the value of the deterioration evaluating linear function. When the deterioration evaluating parameter is thus determined, the correlation between its value and the deteriorated state of the catalytic converter is made clear, making it possible to evaluate the deteriorated state of the catalytic converter more appropriately based on the value of the deterioration evaluating parameter.
The deterioration evaluating parameter may comprise the square or absolute value of the difference between the value of the deterioration evaluating linear function and a predetermined value. Preferably, however, the deterioration evaluating parameter is determined by effecting a low-pass filtering process on the square or absolute value of the difference between each of the time-series data of the value of the deterioration evaluating linear function and a predetermined value as a central value of the value of the deterioration evaluating linear function.
When the deterioration evaluating parameter is determined by effecting the low-pass filtering process on the square or absolute value of the difference, the value of the deterioration evaluating parameter is adequate as a value representing the degree to which the value of the deterioration evaluating linear function varies. As the deterioration of the catalytic converter increases, the value of the deterioration evaluating parameter increases monotonously, clearly indicating a correlation between itself and the deteriorated state of the catalytic converter. Therefore, it is possible to evaluate the deteriorated state of the catalytic converter with high reliability based on the value of the deterioration evaluating parameter.
The low-pass filtering process preferably comprises a filtering process according to a sequential statistic algorithm.
By determining the deterioration evaluating parameter in the filtering process according to the sequential statistic algorithm, the deterioration evaluating parameter can be stored in a reduced memory capacity without the need for a memory for storing many data of the difference and its square or absolute value.
The sequential statistic algorithm may preferably comprise a method of least squares, a method of weighted least squares, a degressive gain method, a fixed gain method, etc.
The deterioration evaluating parameter increases monotonously as the deterioration of the catalytic converter progresses. Therefore, the deteriorated state evaluating means may comprise means for comparing the deterioration evaluating parameter with a predetermined threshold to determine whether the catalytic converter is deteriorated to an extent corresponding to the threshold or not.
Depending on a change in the rate of flow of the exhaust gas entering the catalytic converter, it may not be possible to obtain the value of the deterioration evaluating linear function or the value of the deterioration evaluating parameter which is suitable for evaluating the deteriorated state of the catalytic converter. Therefore, the deteriorated state evaluating means may comprise means for determining whether the deteriorated state of the catalytic converter is to be evaluated or not depending on a change in the rate of flow of the exhaust gas entering the catalytic converter.
Specifically, when the rate of flow of the exhaust gas is maintained stably at a constant level, since disturbance are small, the output of the exhaust gas sensor is likely to be kept stably at the target value steadily according to the feedback control process such as the sliding mode control process. In such a situation, even when the deterioration of the catalytic converter has progressed, the value of the deterioration evaluating linear function is apt to be accumulated near a certain value, making it difficult to distinguish the catalytic converter from a brand-new catalytic converter.
To alleviate such a drawback, the deteriorated state evaluating means may comprise means for failing to evaluate the deteriorated state of the catalytic converter if the rate of flow of the exhaust gas entering the catalytic converter is maintained at a substantially constant level, and evaluating the deteriorated state of the catalytic converter if the rate of flow of the exhaust gas entering the catalytic converter is not maintained at the substantially constant level.
Since the deteriorated state of the catalytic converter is evaluated only when the rate of flow of the exhaust gas changes properly, i.e., when the rate of flow of the exhaust gas undergoes a certain variation, the evaluation of the deteriorated state of the catalytic converter is made highly reliable.
The deteriorated state evaluating means may comprise means for failing to determine the deterioration evaluating parameter if the rate of flow of the exhaust gas entering the catalytic converter is maintained at a substantially constant level, and determining the deterioration evaluating parameter if the rate of flow of the exhaust gas entering the catalytic converter is not maintained at the substantially constant level. With this arrangement, the deteriorated state of the catalytic converter can be evaluated using only the deterioration evaluating parameter that is calculated while the rate of flow of the exhaust gas is appropriate, and the evaluation of the deteriorated state of the catalytic converter is made highly reliable.
The air-fuel ratio manipulated variable generating means may comprise estimating means for sequentially determining data representing an estimated value of the output of the exhaust gas sensor after a dead time of a system ranging from a position upstream of the catalytic converter to the exhaust gas sensor, and means for generating the manipulated variable using the data determined by the estimating means.
Alternatively, the air-fuel ratio manipulated variable generating means may comprise estimating means for sequentially determining data representing an estimated value of the output of the exhaust gas sensor after a total data time which is the sum of a dead time of a system ranging from a position upstream of the catalytic converter to the exhaust gas sensor and a dead time of a system comprising the air-fuel ratio manipulating system and the internal combustion engine, and means for generating the manipulated variable using the data determined by the estimating means.
The system ranging from the position upstream of the catalytic converter to the exhaust gas sensor, i.e., a system for generating the output of the exhaust gas sensor from the air-fuel ratio of the exhaust gas determined by the manipulated variable (hereinafter referred to as xe2x80x9cobject exhaust systemxe2x80x9d), generally has a relatively long dead time due to the catalytic converter included in the object exhaust system. When the rotational speed of the internal combustion engine is comparatively low, the system comprising the air-fuel ratio manipulating system and the internal combustion engine, i.e., a system for generating the air-fuel ratio of the exhaust gas entering the catalytic converter from the manipulated variable (hereinafter referred to as xe2x80x9cair-fuel ratio manipulating systemxe2x80x9d), also has a relatively long dead time. These dead times should preferably be compensated for as they may possibly adversely affect the process of converting the output of the exhaust gas sensor to the target value.
The estimating means determines data representing the estimated value of the output of the exhaust gas sensor after the dead time of the object exhaust system, or data representing the estimated value of the output of the exhaust gas sensor after the total data time which is the sum of the above dead time and the dead time of the air-fuel ratio manipulating system, and the manipulated variable is generated using the data determined by the estimating means.
Because the effect of the dead time can thus be compensated for, the process of converting the output of the exhaust gas sensor to the target value can well be performed. As a result, the deteriorated state of the catalytic converter can be evaluated while well maintaining the desired purifying capability or performance of the catalytic converter.
With the estimating means thus employed, the air-fuel ratio manipulated variable generating means comprises means for generating the manipulated variable to converge the estimated value of the output of the exhaust gas sensor represented by the data determined by the estimating means to the target value, according to the sliding mode control process. The process of converting the output of the exhaust gas sensor to the target value is thus performed highly stably, and hence the desired purifying capability of the catalytic converter can be maintained stably.
The data representing the estimated value of the output of the exhaust gas sensor can be generated using the output of the exhaust gas sensor and the manipulated variable generated in the past by the air-fuel ratio manipulated variable generating means or the detected value of the air-fuel ratio of the exhaust gas upstream of the catalytic converter which depends on the manipulated variable.
The sliding mode control process comprises an adaptive sliding mode control process.
The adaptive sliding mode control process is a combination of an ordinary sliding mode control process and a control law referred to as an adaptive law (adaptive algorithm). When the manipulated variable is generated according to the adaptive sliding mode control process, the reliability of the manipulated variable is increased, allowing the output of the exhaust gas sensor to be converged to the target value with a quick response. The effect of a simple disturbance other than the deteriorated state of the catalytic converter on the value of the deterioration evaluating linear function determined depending on the switching function for the sliding mode control process is suppressed. Consequently, the reliability of the evaluation of the deteriorated state of the catalytic converter based on the deterioration evaluating parameter which represents the degree to which the value of the deterioration evaluating linear function varies is increased.
The manipulated variable generated by the air-fuel ratio manipulated variable generating means comprises a target air-fuel ratio for the exhaust gas entering the catalytic converter, and the apparatus further comprises an air-fuel ratio sensor disposed upstream of the catalytic converter for detecting the air-fuel ratio of the exhaust gas entering the catalytic converter, the air-fuel ratio manipulating means comprising means for manipulating the air-fuel ratio of the air-fuel mixture according to a feedback control process to converge the output of the air-fuel ratio sensor to the target air-fuel ratio.
With the manipulated variable comprising the target air-fuel ratio for the exhaust gas entering the catalytic converter, the output of the air-fuel ratio sensor which detects the air-fuel ratio of the exhaust gas entering the catalytic converter, i.e., the detected value of the air-fuel ratio, is feedback-controlled at the target air-fuel ratio. Therefore, the output of the exhaust gas sensor can well be converged to the target value, and hence the desired purifying capability of the catalytic converter can well be maintained.
The manipulated variable may comprise a corrective quantity for the amount of fuel supplied to the internal combustion engine, for example, other than the target air-fuel ratio. The air-fuel ratio of the air-fuel mixture may be manipulated depending on the manipulated variable according to a feed-forward control process based on the manipulated variable. For maintaining the optimum purifying capability of the catalytic converter and evaluating the deteriorated state of the catalytic converter, the exhaust gas sensor should preferably be an oxygen concentration sensor, i.e., an O2 sensor, and a target value therefor should preferably be a constant value.